Heat pumps may operate as heaters and/or air conditioners. The heat pumps utilize refrigerant to transfer heat from one location to another depending on the mode they are operating in. When operating as a heater, they transfer heat from outside to an area to be heated. When operating as an air conditioner, they transfer heat from an area to be cooled to outside. The heat pumps operate more efficiently when the outside temperature is closer to the operational temperature of the refrigerant. As the temperature of the atmosphere fluctuates vastly over the seasons the efficiency of the unit is not optimal.
FIG. 1 illustrates a simple cross-sectional view of an example area 100 of earth. The cross-sectional view includes an upper layer 110 that may be, for example, top soil. Under the upper layer 110 is a geological formation 120 that may be, for example, granite, bedrock or sand. Below the water table 130 for the area 100, groundwater fills the cracks and spaces in the geological formation to form aquifers 140. For clarity, the geological formation is the area both above 120 and below 140 the water table 130 and the aquifer 140 is the area below the water table 130.
Geological formations 120, 140 remain at a constant temperature throughout the year due to being insulated from seasonal temperature variations in the atmosphere. In addition, geological formations 120, 140 can store significant amounts of thermal energy. In general, these two properties of geological formations 120, 140 make geothermal systems a prime technology for increasing the efficiency and reducing the cost associated with heating and cooling buildings throughout the seasons. Geothermal systems are typically connected to heat pumps associated with a buildings heating, ventilation and/or air conditioning (HVAC) system so that the geologic formation 120, 140 acts as a heat source or heat sink for the heat pump depending on whether the heat pump is being used for heating or cooling. Geothermal systems may also be connected to heat exchangers to provide direct heating or cooling without the need for a heat pump.
FIG. 2 illustrates an example of a typical closed loop geothermal system 200. The system 200 includes a borehole 210 drilled into the geological formation 120, 140. Tubing 220 is connected to an HVAC system (e.g., heat pump) for a building and is placed down the borehole 210. The tubing 220 circulates fluid from the buildings HVAC system to transfer heat to the geological formation 120, 140 or receive heat from the geological formation 120, 140. A material impermeable to water flow (e.g., clay, bentonite) 230 is pumped around the tubing 220 to prevent surface water from entering the borehole 210 and potentially contaminating the groundwater, to improve thermal conduction, and to mitigate leaks of fluid from the closed loop system 200. The efficiency of the closed loop system 200 is limited to the amount of thermal energy that can be transferred to/from the geologic formation 120, 140 as the fluid from the HVAC system traverses the tubing 220 within the borehole 210. Because the closed loop system 200 segregates the groundwater in the geological formation 120, 140 from fluid (e.g., surface water) entering and exiting the borehole 210 and because they do not deplete the water supply in the geologic formation 120, 140, they are generally allowable by government entities.
FIG. 3 illustrates an example of a typical open loop geothermal system 300. The system 300 includes a borehole 310 drilled into the geological formation 120, 140 and a pump 320 located with the borehole 310 to extract groundwater 360 from the aquifer 140. The pump 320 pumps the groundwater 360 out of the ground via tubing 330 and the groundwater 360 is somehow circulated through a heat exchanger 340 and then discharged back into the geological formation 120, 140. Tubing 350 connects an HVAC system (e.g., heat pump) for a building to the heat exchanger 340 so that fluid from the buildings HVAC system can be circulated through the heat exchanger 340 in order to transfer heat to the groundwater 360 or receive heat from the groundwater 360. The circulation of the groundwater 360 through the heat exchanger 340 generally provides better performance characteristics than the closed loop system 200. However, open loop systems are often not permitted by governmental entities due to the possibility of surface water infiltration, groundwater depletion through discharge into sewers, and contamination of the aquifer 140.
The thermal conductivity of the geologic formation 120, 140 determines the number and depth of the boreholes 210 drilled for adequate heat transfer in a closed loop system 200. The drilling costs of the boreholes 210, 310 are the major capital cost in the closed loop geothermal system 200. Accordingly, there exists a need to circumvent this thermal conductivity limitation to increase the capacity of a borehole 210, and reduce the number and depth of boreholes 210 required for adequate heat transfer. This, in turn, will reduce capital costs and return on investment, leading to higher market acceptance.